This invention relates to a method and apparatus for digital video signals in which data produced on encoding video signals by the MPEG system may be directly recorded and a reproduced picture of high picture quality may be produced on varying-speed reproduction.
Recently, developments of a digital video tape recorder (digital VTR) in which video signals are converted into digital signals and transformed by discrete cosine transform (DCT) so as to be variable length encoded by e.g., Huffman code and recorded on a magnetic tape by an inclined azimuth recording system using a rotary head, are proceeding briskly. Such digital VTR can be set to a mode of recording video signals of the current television system, such as the NTSC system, referred to herein as SD mode, or to a mode of recording high-definition television signals (HDTV signals), referred to herein as HD mode.
With the SD mode, video signals are recorded after compression to digital video signals of approximately 25 Mbps, whereas, with the HD mode, the HDTV signals are recorded after compression to digital video signals of approximately 50 Mbps.
With the conventional digital VTR, it is contemplated to directly record input digital video signals, that is input data, on a magnetic tape, or to reproduce and directly output data recorded on a magnetic tape. That is, by adding the function of directly recording/reproducing digital video signals to the conventional digital VTR, it becomes unnecessary to decode the input digital video signals for reproducing e.g., HDTV signals, and to re-encode the HDTV signals by a pre-set encoding system for recording on the magnetic tape, thereby eliminating hardware waste.
Specifically, if digital video signals produced on encoding video signals by MPEG system, that is a moving picture encoding system standardized by the Working Group (WG) 11 of the Sub-Committee (SC) of Joint Technical Committee (JTC) of the International Electrical Conference (IEC) and International Standardization Organization (ISO), or digital video signals reproduced from an optical disc, are supplied to a digital VTR, it is convenient for the digital VTR to have the function of directly recording/reproducing these digital video signals.
The advanced television (ATV) system, a digital broadcasting employing the MPEG system as the encoding system, is now explained.
In FIG. 1, which is a block diagram showing the construction of a transmission system of the ATV system, 101 is a video encoder and 102 an audio encoder. The video encoder 101 is fed via an input terminal 103 with video signals of the HDTV system, while the audio encoder 102 is fed via an input terminal with an audio signal.
The video encoder 101 encodes the input HDTV signals by the MPEG system by way of data compression. That is, the encoder 101 encodes the HDTV signals using a high efficiency encoding system consisting in the combination of DCT and motion compensated prediction coding by way of data compression. The video encoder 101 outputs data of I-picture obtained on intra-field or intra-frame coding, data of the P-picture obtained on forward predictive coding and data of the B-picture obtained on bi-directional coding, in a pre-set sequence, as shown in FIG. 2. The frame and the field are occasionally referred to herein as picture. The I-picture is produced by independent DCT without exploiting correlation with other pictures. With the P-picture, motion compensated predictive coding is carried out from the temporally previous I-or P-picture and the resulting difference signal, that is the prediction error, is transformed by DCT. With the B-picture, motion compensated predictive coding from the temporally forward or backward I- or P-picture is carried out and the resulting difference signal is transformed by DCT. The period of the I-picture is termed the group-of-pictures (GOP). In the present case, M=3 and N=9.
A transport encoder 106 generates a packet from video data encoded by the video encoder 101, audio data encoded by an audio encoder 104 and subsidiary information from an input terminal 107.
In FIG. 3, showing a packet construction, the length of a transmitted packet is 188 bytes. At the leading end of the packet are a ring header having a fixed length of 4 bytes and an adaptation header of a variable length, followed by transmission data consisting of video data or audio data.
Referring to FIG. 1, the packet generated by the transport encoder 106 is supplied to a channel modulator 108 which then modulates the packet using a carrier of a pre-set frequency. An output of the channel modulator 108 is issued at an output terminal 109.
With the ATV system, HDTV signals can be transmitted at a rate of e.g., 10 Mbps which is lower than the recording rate of approximately 25 Mbps for the SD mode of the digital VTR. Thus the signals transmitted by the ATV system can be directly recorded on the digital VTR. By directly recording the transmitted signals on the digital VTR, there is no necessity of decoding HDTV signals from the transmitted signals and inputting the decoded signals to the digital VTR so that waste in hardware is eliminated. In addition, a longer recording time may be achieved by SD mode recording.
However, if the ATV signals are directly recorded with the SD mode on the digital VTR, varying-speed reproduction cannot be achieved for the following reason.
With the ATV system, compression coding is carried out in accordance with the MPEG system, in which the I-pictures, P-pictures and B-pictures are transmitted, as explained previously. Since the head traverses the tracks on the magnetic tape, it becomes impossible to produce data of continuous pictures. If the data of the continuous pictures cannot be obtained, the data of the P-and B-pictures cannot be decoded. It is only the I-picture that can be decoded. Consequently, the varying-speed reproduction becomes possible only by using data of the I-pictures.
However, if the signals transmitted by the ATV system are directly recorded by the digital VTR, packets including the I-pictures cannot be picked up satisfactorily during varying-speed reproduction. On the other hand, it is not definite in which relative position the data of the I-picture is recorded, so that data of the I-picture corresponding to specified picture portions on varying speed reproduction are dropped and hence the picture of that portion cannot be updated for some time thus deteriorating the picture quality during varying speed reproduction.
The present Assignee proposed a system in which in which at least one of the reproducible areas traced by the head at the maximum varying speed is used as an area for varying speed reproduction, and data of the I-picture is extracted form the input bitstream of the ATV system and repeatedly recorded in each varying speed playback area of a number of tracks of the same azimuth corresponding to the multiple speed of the maximum playback speed and in which the signals of the ATV system are directly recorded in other video areas.
This reproduces the varying-speed playback area during varying speed reproduction and a picture may be formed from data of the I-picture reproduced from this area.
For reproducing a specified area in a digital VTR of the inclined azimuth recording system, designed for recording digital video signals on the magnetic tape by the rotary head, the tape running system needs to be controlled to apply phase lock. Although tracking servo is applied during normal reproduction by utilizing tracking control signals (ATF signals) recorded on each track, tracking control utilizing the ATF signals cannot be done in case of varying speed reproduction since the head traverses the tracks on the magnetic tape.